A typical modern multiple speed automatic transmission includes a combination of planetary gear assemblies and selectively engaged friction clutches and to achieve a plurality of gear ratios.
Increasingly demanding economic and performance goals continue to encourage automatic transmission research and development. A result of this effort has been an increase in the number of available forward gears or speed ratios provided by the transmission.
Whereas three and four speed automatic transmissions were once commonplace, considered both suitable and to provide sufficient flexibility and performance, the industry and consumer preference is moving to six, seven and eight speed automatic transmissions.
In such transmissions, the various elements of a plurality of planetary gear assemblies are connected by permanent couplings, selectively connected by clutches and selectively grounded by brakes. Specific combinations of the clutches and brakes are engaged or activated to provide a sequence of numerically related gear ratios and thus speed and torque ratios.
Because they so closely match the power and torque curves of the engine to the vehicle speed and load, such six, seven and eight speed transmissions provide significant performance enhancements and fuel consumption reduction.
Careful study of these transmission configurations, however, reveals that improvements are both possible and desirable. For example, each of the torque transmitting devices, i.e., the clutches and brakes, contribute to frictional losses, referred to as spin losses, when they are not engaged. Two primary factors influence spin losses: the size or torque capacity of the clutch and the instantaneous speed difference across the clutch. A particularly disadvantageous condition arises if the device is a high torque capacity, multiple plate clutch functioning as a brake with a high speed difference across it. Since a brake typically has one half of its friction members connected to ground which are therefore always stationary, the spin losses in such a situation may be relatively significant.
The present invention is directed to reducing frictional or spin losses created by the clutches and brakes of a multiple speed automatic transmission.